1. Field of the Invention
The present invention relates to a transferring apparatus, incorporated in an image forming apparatus such as an electronic copying machine, a printer and a facsimile machine, for transferring a toner image formed on the surface of an electrostatic latent image carrier incorporated in the image forming apparatus onto a sheet such as a transfer sheet.
2. Description of the Prior Art
Two typical apparatuses for performing electrostatic transfer of a toner image formed on the surface of an electrostatic latent image carrier in an image forming apparatus, such as an electronic copying machine, are an apparatus employing a non-contact-type corona transfer method where a corona discharger is used as a means for supplying a charge to the carrier, and an apparatus employing a contact-type bias roller transfer method where a conductive roller is used as the charge supplying means.
In the former transferring apparatus employing the corona transfer method, as schematically shown in FIG. 1, a corona discharger 22 is arranged in such manner that a necessary gap is left between the discharger 22 and a photoreceptor drum 21 which serves as the electrostatic latent image carrier, and a sheet P transported between the rotating drum 21 and the discharger 22 is made to partly adhere to the drum surface by charging the PG,3 sheet P by applying a corona charge of a polarity reverse to that of toner from the back side of the sheet P, and the charged toner on the drum surface is transferred to the sheet P by the Coulomb's force.
In the case of the corona transfer method, however, since the sheet P adhering to the drum 21 due to the working of electrostatic force frequently does not spontaneously separate from the drum surface, it is necessary to provide a separating means. Moreover, since a high voltage is applied to the discharger 22, ozone which is harmful to the human body is generated in a considerable amount.
On the contrary, in the latter transferring apparatus employing the bias roller transfer method, as schematically shown in FIG. 2, a transfer roller 23 made of urethane resin provided with conductivity by being mixed with carbon or alkali metal is arranged so as to be rotatively in contact with the photoreceptor drum 21, and the transfer sheet P admitted between the peripheries of the drum 21 and the transfer roller 23 is pressed by the transfer roller 23 onto the toner adhering to the drum surface and the toner image on the drum surface 21 is transferred onto the sheet P by applying to the axial core of the transfer roller 23 a transfer voltage of a polarity reverse to that of the toner. This method is more advantageous than the above-mentioned corona transfer method in that the generation of ozone is smaller and no sheet separating means is necessary.
In the bias transfer method, however, since the surface of the drum 21 and the surface of the transfer roller 23 are pressed against each other with the sheet P between, it is apt to occur that the toner located at a central portion of the drum 21 is not transferred to the sheet P and that toner scatters and adheres to a peripheral portion of the image transferred onto the sheet.
To solve these problems of the bias roller transfer method, the present applicant proposed in Japanese Patent Application No. H4-284120 an arrangement as schematically shown in FIG. 3, wherein the transfer roller 23 is arranged so that a gap larger than the thickness of the sheet P is left between the photoreceptor drum 21 and the roller 23 in order that the roller 23 remains out of contact with the drum 21 and the sheet P to thereby prevent the occurrence of the above-mentioned problems.
In FIG. 3, 24 represents a resist roller pair for transporting the sheet, 25a and 25b represent guide members for guiding the sheet P to the drum surface, and 26 represents a guide table for guiding to a fixing roller pair (not shown) the sheet P onto which the image has been transferred.
In the case of the above-mentioned prior art, it is considered that the reason why the toner image on the drum surface is appropriately and excellently transferred onto the sheet P in spite of the fact that the transfer roller 23 is separated from the surface of the drum 21 is that a slight corona discharge is generated from the transfer roller 23 toward the rear surface of the sheet P. Actually, excellent test results have been obtained with respect to the transfer performance of the toner image.
However, since nothing supports the sheet P from the rear side because the transfer roller 23 is out of contact with the drum surface, the movement of the sheet P is unstable, thereby causing a variation in contact pressure between the sheet P and the drum surface. As a result, the transferred image is uneven.
That is, in the above prior art, the resist roller pair 24 sends out the sheet P in correspondence with the rotation of the drum 21 at a speed equal to the peripheral speed of the drum surface, so that the sheet P is guided toward the drum along the guide members 25a and 25b. After the front end of the sheet P passes by the end portion of the lower guide member 25b and abuts the drum surface, the sheet P moves at a speed equal to the peripheral speed of the drum surface while being in close contact with the transfer area on the drum surface by being bent along the surface of the drum 21, and by the time the sheet P is separated from the drum surface, the toner image is transferred onto the sheet which is in close contact with the transfer area on the drum surface by the working of the transfer roller 23 from the rear side of the sheet P.
The sheet P which has been separated from the drum surface, although depending on its length, moves toward the downstream side on the sheet conveying path while being weighed down by its own dead load, and after reaching the upper surface of the guide table 26, it is separated from the resist roller 24 and transported along the guide table 26 in accordance with the rotation of the drum 21. As described above, the sheet P bends at a portion just in front of the portion of the sheet P which abuts the drum surface and at a portion at which the sheet P comes into contact with the guide table 26, and the resiliency of the sheet P generated by the bend works as the contact pressure on the portion of the sheet P which is in contact with the drum surface.
The conveying speed of the sheet P and the inclination angles of the guide surfaces of the guide members 25a and 25b are set to fit the conditions to make the sheet P to be in close contact with the drum surface while being conveyed at a speed equal to the peripheral speed of the drum surface. In actuality, however, the resiliency of the sheet P varies according to environmental conditions such as the temperature and humidity around the transfer area.
For this reason, it is unavoidable that the sheet P is not in contact with the drum surface with a necessary contact pressure because the sheet is excessively bent, that is, excessively weighed down and that the position relationship between the toner image on the drum 21 and the sheet P is shifted because of accumulative errors caused in the sheet feeding speed of sheet feeding rollers provided at a plurality of positions in a paper feeding mechanism.
Such error factors relating to paper feeding cause no problems in the case of the conventional contact-type transfer roller since the roller rotates in synchronism with the drum 21 to restrict the conveying speed of the sheet P. However, when the transfer roller is out of contact with the drum 21 like in the above prior art, since no means is provided for forcibly restricting the conveying speed and the transfer timing of the sheet P, it is impossible for structural reasons to prevent the transferred image from being uneven.
The sheet P which has been separated from the drum 21 is pulled by the fixing roller pair after the separation. At this time, since the peripheral speed slightly differs between the fixing roller pair and the resist roller pair 24, the sheet P may excessively be pulled by the fixing roller pair or it may be bent between the drum 21 and the fixing roller pair. However, regarding this point, normally, the diameters of the rollers constituting the resist roller pair 24 and the fixing roller pair are each set so that the sheet P is slightly bent between the roller pairs.
Moreover, when the sheet P is short, the rear end thereof may be separated from the resist roller pair 24 while transfer is still being performed. For such a case, utilizing the fact that the sheet P is attracted to the drum surface by the attraction caused by the electrostatic force, the rollers and guide members are arranged in a manner such that the sheet P is conveyed to the fixing roller pair without any position shift or disorder by making the drum surface attract a part of the sheet P by use of the attraction by the electrostatic force.
In the arrangement of the above prior art, however, although the movement speed of the sheet P and the peripheral speed of the drum surface are maintained coincident with each other when the sheet P is caught between the resist rollers 24 as shown in FIG. 3, after the rear end of the sheet P is separated from the resist roller pair 24 as shown in FIG. 4, since the sheet P comes into close contact with the upper guide member 25a due to its resiliency, and its movement speed slightly changes, the electrostatic latent image formed on the drum surface is transferred, at the rear portion of the sheet P, in a condition where the position is slightly shifted.
Moreover, if the angle of inclination of the upper guide member 25a is displaced to an angle which is nearly horizontal as shown in FIG. 5 so that the rear end portion of the sheet P separated from the resist roller 24 does not interfere with the upper guide member 25a, the admission angle of the sheet P to the drum surface is displaced to change the contact condition of the sheet P with the transfer area of the drum surface, thereby changing the image density. Therefore, when a high-quality image is required, it is difficult to realize a sufficient transfer accuracy only by exactly setting the arrangement of the rollers and guide members or changing the position of the guide members.